Truss screeds are widely used to level and preliminarily finish freshly poured concrete. A typical truss screed includes at least one screed plate and a triangular truss frame that supports the screed plate and other components of the machine. The ends of the screed plate are configured to be supported on an upper edge of a form surrounding a slab of freshly-poured concrete. In use, the screed is pulled along the form, either by a manually operated or power operated winch, so that the screed plate pushes the formed concrete ahead of it to level the concrete. Oftentimes, the screed is sectional. That is, it consists of modular sections that are connected to one another in an end-to-end fashion. Sections can be added or removed as desired to change the effective length of the screed, thus permitting the screed to be used on concrete slabs of variable widths. The relative angular orientation of the various sections can also be adjusted to alter the profile of the leveled concrete, e.g., to impart a crown or a slant to the leveled surface.
Vibrational forces can be imparted to the screed plate during a concrete leveling operation. Vibration during screeding helps settle and densify the concrete. Vibrational screeding also removes air voids from the concrete and brings excess water and fine layers of concrete aggregated to the surface, hence partially finishing the leveled concrete. Vibrational forces are typically imparted using an exciter shaft that is located near the screed plate and that is driven to rotate via a motor such as an internal combustion engine. The exciter shaft supports eccentric weights that generate vibrations upon exciter shaft rotation. The vibrations are transmitted to the screed plate through the exciter shaft and its bearings.
The triangular truss frame is typically formed of several beams extending the length of the screed, or extending the length of each module and are connected end-to-end to form a longer effective length of the screed. The beams are generally arranged in a triangle when viewed from an end of the truss frame, with an upper beam at an apex of the triangle and a first and second lower beam forming the lower two corners of the triangle. A series of struts are arranged, generally horizontally, to hold the two lower beams in a fixed, parallel relationship, and a series of struts extend between each lower beam and the upper beam to hold the upper beam in a fixed parallel relationship relative to the lower beams. Such an arrangement is shown in U.S. Pat. No. 6,457,902.
Vibrating truss screeds used for concrete construction have traditionally consisted of either 1) a welded steel or aluminum frame, or 2) a bolt-together assembly of beams, rods and struts. A third design utilizes cast truss sections that are attached to the beams with fasteners. Problems arise with welded truss sections because of the heat-affected zones. The vibration exerted by the machine leads to shear failure at a certain number of cycles. Castings attempted to solve the problem, but inadvertently caused a cost increase.
It would be an advance in the art if a truss frame were provided that is light weight, cost effective, sturdy and stiff, which allows for ease of use and modification as the size of various jobs requiring the screed changes.